Unformatted text preview: removed from memory.
If you’re using any version of Microsoft Windows, press Ctrl-Alt-Delete and click on the Task Manager
button. There are tabs labeled Applications and Processes. Applications are the names of complete programs
currently running, such as Windows Explorer or Microsoft Visual C++. When you click on the Processes
tab, you see 30 or 40 names listed, often some you might not recognize. Each of those processes is a
small program running independent of all the others. Note that each has a PID (process ID), and you can
continuously track the amount of CPU time and memory it uses. Most processes run in the background.
You can shut down a process somehow left running in memory by mistake. Of course, if you shut down
the wrong process, your computer may stop running, and you’ll have to reboot. Multitasking
A multitasking operating system is able to run multiple tasks at the same time. A task is deﬁned
as either a program (a process) or a thread of execution. A process has its own memory area
and may contain multiple threads. A thread shares its memory with other threads belonging to
the same process. Game programs, for example, often use individual threads to simultaneously For More BS-IT Books, Notes & Assignments visit: www.bsit.zxq.net 2.1 General Concepts
Downloaded From: www.bsit.zxq.net 35 control multiple graphic objects. Web browsers use separate threads to simultaneously load
graphic images and respond to user input.
Most modern operating systems simultaneously execute tasks that communicate with hardware, display user interfaces, perform background ﬁle processing, and so on. A CPU can really
execute only one instruction at a time, so a component of the operating system named the scheduler allocates a slice of CPU time (called a time slice) to each task. During a single time slice,
the CPU executes a block of instructions, stopping when the time slice has ended.
By rapidly switching tasks, the processor creates the illusion they are running simultaneously.
One type of scheduling used by the OS is called round-robin scheduling. In Figure 2–4, nine
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This note was uploaded on 11/29/2013 for the course CSE 451 taught by Professor Hussein during the Winter '13 term at Fatih Üniversitesi.
- Winter '13